In seismic exploration of the earth, as practiced in the search for oil, gas and other minerals, a burst of low frequency energy is imparted to the earth at a first location. For example, in exploration of the seabed, a ship towing one or more compressed air "guns" will fire a "shot" at regular intervals, e.g. every 10 seconds. A "streamer" of "hydrophones" trailing behind the ship detects return of the acoustic energy of the "shot" after reflection from interfaces between rock layers of the seabed. Similarly, in land-based exploration, a charge of explosives is detonated or a heavy weight is dropped to impart a pulse of energy into the earth. A "spread" of "geophones" detects return of the acoustic energy to the surface of the earth after reflection at interfaces between the subterranean rock layers.
The signals detected at the surface are conventionally recorded with respect to each of the detectors and if graphed display amplitude data varying with time. "Wavelets" corresponding to the pulse of energy having been reflected from a subterranean interface are received and appear in the seismic "trace" recorded with respect to each individual geophone. There is extensive art relating to the processing of these "traces" to yield seismic records or "seismograms", which represent with more or less accuracy cross-sectional "pictures" depicting the subterranean structure of the earth. Such a cross-sectional slice through the earth underneath the line of geophones provides definition of the interfaces between the various layers of different types of rock. Certain typical structures are known to be more likely to contain oil and gas than other types of structures.
The traces which are recorded depict the amplitude of the output signals of the geophones versus time. The traces are readily ordered according to the "offset", that is, the distance of the corresponding geophones from the shotpoint. In order to generate a "picture" of the subterranean structure, that is, to provide a cross-section through the earth in two spatial dimensions (depth in the earth versus displacement from the location of the shot along the surface) from the amplitude versus time data which is recorded by each trace, it is essential to know the velocity of the pulse of energy in the various rock layers. This velocity information is essential in translating the time information provided by the geophones into distance information, indicative of the distance of the interfaces between the rock layers from the surface.
The relative velocity of sound in the various layers also determines the way in which a particular wave or ray will travel between the source and each geophone. According to Snell's Law, the angle made by a ray (whether of light or sound) at an interface between different media (e.g. water and air in the case of a light ray, or two different rock layers in the case of a seismic wave of acoustic energy) is determined by the relative velocity of the ray in the two media. Accordingly, if one wishes to trace a hypothetical ray from a particular source to a particular geophone, it is essential to know or make assumptions concerning the velocity of sound in the various rock layers.
Finally, the velocity of sound in a particular layer is indicative of the type of rock of the layer, which is itself of interest.
Given valid velocity data and reasonable assumptions concerning the shapes of the interfaces, that is, the thickness of the various layers, one can construct a synthetic seismic record. This can be compared to the actual seismic record, allowing the accuracy of the model to be evaluated. The assumptions can be changed as needed, the model recalculated, and so on until a reasonably accurate model of the subterranean "structure" has been reached.
The normal practice in the seismic art is to assume that the velocity of sound is either constant in the various layers or varies linearly with depth in the structure. That is, it is usually assumed that as the wave travels further into the earth, its velocity increases as the structure becomes denser due to its greater depth in the earth. These assumptions are used in modeling the earth, e.g. in generating a synthetic seismogram. Moreover, the seismic processing techniques used normally require the assumption that the structures are layered regularly. It will be appreciated, of course, that these assumptions are rarely if ever correct and that more accurate assumptions would yield improved results. It is an object of this invention to provide a velocity model more likely to correspond to reality, and which permits the employment of more complex models of the subterranean structure of the earth.